Fluid flows with temperature gradients due to internal heating or heated/cooled walls are examples of thermal flows, which form an integral part of convection heat transfer. Internal heating may result from fluid friction (viscous dissipation), irradiation (thermal radiation), and/or chemical reactions (reactive flows). When the fluid temperature differs from the wall temperature, the thermal wall conditions are commonly expressed as Tw = const (i.e., isothermal wall) or qw = const (i.e., constant wall heat flux). Convection heat transfer is heat conduction, that is, an energy diffusion process, in a moving fluid. Combining heat conduction in solids with thermal convection in a fluid is called a conjugate heat transfer problem as it might occur in heat pipes, heat exchangers, fin cooling, tribology, porous media flow, and so forth. Mixed thermal convection, that is, simultaneous free and forced convection heat transfer, has to be considered when the buoyancy force is of an order of magnitude comparable to the inertia force. In free and in mixed convection problems a heat source or sink affects the fluid density, typically near a heated or cooled wall, and thus the momentum equation depends via the body force term on the heat transfer equation. Such a two-way coupling may also occur when other fluid properties are temperature-dependent.

Nonisothermal flow problems are typically subdivided into external flows and internal flows where the flow regime is either laminar or turbulent, and the flow is single phase or multiphase.